Genetic testing has been increasingly utilized during prenatal care. Beginning at approximately nine weeks of gestation, the health and genetic status of the fetus can be examined by a variety of prenatal diagnostic techniques. Two approaches, ultrasonography and the measurement of .alpha.-fetoprotein in maternal serum, are increasingly being used for diagnosis in the absence of other indications. Ultrasonography allows determination of gestational age, identification of grossly abnormal development, identification of major developmental abnormalities of the head, spine, head, gastrointestinal tract, kidney and skeleton, and detection of multiple gestations (Diagnostic Ultrasound: Applied to Obstetrics and Gynecology, 2nd Ed., Sabbagha, R. E. (ed.), J. B. Lippincott Co., Philadelphia, 1987, p.2). The maternal serum .alpha.-fetoprotein level is used to screen pregnancies for several disorders: Elevated values raise the possibility of a fetal neural tube defect and low levels raise the possibility of trisomy 21, or, less frequently, trisomy 18. In either case, further tests are often required to confirm that abnormal .alpha.-fetoprotein level is the result of a congenital disorder rather than a false positive result.
Chorionic villus sampling and amniocentesis are both methods which are used to provide a sample of fetal cell for cytogenetic and, when indicated, metabolic or molecular analyses. Analysis of fetal cells currently allows for the prenatal diagnosis of more than 200 Mendelian disorders, most of the major chromosomal disorders, and some multifactorial disorders. In chorionic villus sampling, a biopsy of mixed fetal and maternal cells is obtained from the chorion frondosum using a transcervical or transabdominal approach. The fetal cells are then mechanically separated for analyses. Amniocentesis relies on procuring a sample of amniotic fluid from the pregnant woman, which is then separated into the cellular component (used for cytogenetic, biochemical, and molecular analyses) and the fluid supernatant (used to measure the .alpha.-fetoprotein concentration). Cytogenetic karyotype analysis on cells obtained through either procedure is a routine recommended screen in all pregnancies to mothers age 35 or older and is now a major biomedical industry. Other tests on cells obtained through these procedures are performed much more rarely, and only when specifically indicated, not as a screening procedure. Fetal cells can also be obtained by cordocentesis, or percutaneous umbilical blood sampling, although this technique is technically difficult and not widely available (see Erbe, R. W., 1994, in: Scientific American Medicine, Volume 2, section 9, chapter IV, Scientific American Press, New York, pp 41-42). In the majority of cases prenatal diagnostic studies are a routine component of standard medical care in individuals with no specific risks other than age.
However, in many instances, individuals or couples seek genetic information because a relative has a genetic disorder or condition known to have a genetic component. The reproductive alternatives available to a couple or individual at risk depend on the specific disorder and the availability of tests for the disorder.
Mutations in the fibroblast growth factor receptor (FGFR) gene family (designated FGFR1, PGFR2, and FGFR3) have recently been shown to underlie several dominantly inherited disorders of bone development. FGFR1 mutations have been shown to produce Pfeiffer syndrome, FGFR2 mutations have been shown to produce Crouzon, Jackson-Weiss, Pfeiffer and Apert syndromes, and FGFR3 mutations have been shown to cause achondroplasia, thanatophoric dysplasia types 1 and 2, hypochondroplasia, and Crouzon syndrome with acanthosis nigricans and FGFR3-associated coronal synostosis (reviewed in Wilkie, A. O. M., et al., 1995, Current Biol. 5:500-507; Mulvihill, J. J., 1995, Nature Genet. 9:101-103). All of the mutant disease-causing alleles found in the three receptor types are dominant. And most appear sporatically as a result of new mutations, not in families at known risk for the disease. Mutations at different FGFR loci can give the same disease phenotype and a given allele can produce different disease phenotypes. The majority of the different alleles appear to alter a common structure, the receptor dimer. This suggests that mutations in all three types of FGFR may share a common pathophysiological mechanism. However, no defect in receptor function has previously been directly demonstrated for these mutations.
Previous studies have used conventional knockout mice (e.g., Colvin, J. S., et al., 1996, Nature Genet. 12:390-397) or in vitro studies of transfected chimeric receptors (e.g., Webster, M. K., and Donoghue, D. J., 1996, EMBO J. 15:520-527; Galvin, B. D., et al., 1996, Proc. Natl. Acad. Sci. U.S.A. 93:7894-7899) to examine FGF receptors. However, these studies do not fully mimic the native or disease state.
Most tissues express multiple FGFR types, including splice variants of each, and most FGF ligands discriminate poorly between the receptor types. Both hetero- and homo-receptor dimers contribute to signaling (Johnson, D. E., and Williams, L. T., 1993, J. Biol. Chem. 267:1470-1476). Model negative dominant mutations in one receptor class can create a dosage-dependent inhibition of all FGF signaling, even though many wild-type FGF receptors of the same and different type are expressed. This kind of interaction between the FGFR types has been defined in transfected cultured cells, injected Xenopus oocytes, embryos, and the epidermis of transgenic mice.